Titanium dioxide (TiO2), or titania, is a wide bandgap semiconductor material that is inexpensive, chemically stable, and has negligible absorption in the visible region. A particularly intriguing property of this material is its tendency to generate electron-hole pairs upon irradiation with UV light. This process provides a means of inducing chemical reactions at the surface of the material. TiO2 is often utilized as a photocatalyst, an electrode in dye-sensitized solar cells, as a gas sensor, and as a decontamination agent.
The generation of electron-hole pairs and the subsequent transport of charge through the particle network are significant factors in determining whether or not TiO2 is an efficient catalyst or electrode (Diebold U, Surf. Science Rep., 2002; 48; 53-229). The efficiency of titania as a catalyst or electrode is maximized when charge transport is significantly faster than hole-charge recombination. For example, when TiO2 is used as an electrode in dye sensitized solar cells, charge transport has been found to be highly dependent on the morphology associated with the particle network. The number of inter-particle connections, or the lack thereof, can lead to hole-charge recombination (Zhu K, Neale N R, Miedaner A, Frank A J, Nano Letter 2007; 7:69). One approach to mitigating insufficient inter-particle connections has been the use of one-dimensional TiO2 nanostructures, which maximize charge transport along the major dimension.
Conventional approaches used to synthesize TiO2 nanorods include electrochemical anodization, hydrothermal synthesis, and template-assisted synthesis.
Hydrothermal synthesis, in particular, has been found to be a highly versatile approach to fabricating various types of nanocrystals. This technique is scalable and provides experimental control of temperature within the pressurized vessel. Numerous attempts to fabricate TiO2 nanorods and nanotubes have been reported using this technique. These approaches primarily involve the treatment of pre-synthesized TiO2 nanoparticles with sodium hydroxide (NaOH), resulting in the formation of lamellar sheets via the rupture of Ti—O—Ti bonds (Kasuga T, Hiramatsu M, Hoson A, Sekino T, Hiihara K. Adv. Materl. 2004, 16, 2052). Subsequent washing steps with water and acid, which remove the electrostatic charge within the sheets, were found to lead to the formation of nanotubes and nanorods. Zhang et al. describe a simple chemical/hydrothermal approach to fabricating TiO2 nanowires with diameters from 30 to 45 nm and lengths of several micrometers (Zhang X Y, Li G H, Jin Y X, Zhang Y, Zhang J, Zhan L D. Chem. Physics Lett. 2002; 365:300).
In conjunction with the hydrothermal approach, several additives have also been explored as a means of inducing the formation and elongation of TiO2 nanotubes, nanorods, and nanowires during the hydrothermal process. These additives include oleic acids, oleylamine, and tetraalkylammonium cations (Zhang Z, Zhong X, Liu S, Li D, Han M. Angew Chem. 2005; 44:3466) (Seo J W, Jun Y W, Ko S J, Cheon J. J. Phs. Chem. B 2005; 109:5389) (Chemseddine A, Mortiz T. Eur. J. Inorg. Chem 1999; 1999:235). Oxalic acid has been used as a means of complexing with metal cations while also promoting the formation of linear crystal structures in other metal oxides, including MgO and WO3 (Mastuli M S, Jamarulzaman N, Nawawi M A, Mahat A M, Rusdi R, Kamarudin N, Nanoscale Res. Lett. 2014; 9:134) (Rao C N R, Natarajan Sm Vaidhyanathan R. Angew. Chem., Int. Ed 2004; 43:1466). In Mastuli et al., it was postulated that the formation of a metal cation-oxalate complex precedes the subsequent formation of a linear polymer network of Mg—C2O4, before forming the eventual MgO nanowires. Dambournet et al. describe the use of oxalate anions to complex with titanium atoms, which yielded titanium oxalate hydrate, Ti2O3 (H2O) (C2O4)—H2O (Dambournet D, Belharouak I, Amine K. Chem. Mater. 2010; 22:1173). Upon annealing at 300° C., egg shaped TiO2 particles were obtained. A metal:oxalate ratio of 1:2, was determined to be optimal for the formation of rutile crystals.
As described in U.S. Pat. No. 9,085,467 TiO2 has various crystalline shapes. The most common crystalline phases of TiO2, anatase, rutile and brookite, exhibit different chemical/physical properties, such as stability field, refraction indexes, chemical reactivities and behavior to irradiation with electromagnetic radiation. The use and performance of TiO2 depends greatly on its crystalline phase, on its morphology and on the dimensions of the particles, as reported, for instance, by X. Chen and S. S. Mao in J. Nanosci. Nanotechnol, 6(4), 906-925, 2006. The phase composition, the shape of the crystals and the dimensions of the particles exert an enormous influence over the chemical/physical, mechanical, electronic, magnetic and optical properties of the end product.
It is desirable to provide a process to produce nanocrystalline, TiO2 particles with a desired shape and high levels of specific surface.